Bird Flight: Tail-Rudders and the Centre of Gravity
How a bird steers with its tail-feathers and descends toward its centre of gravity
Leonardo analyses how the tail-feathers, the 'rudders' of a bird's wings, direct it up or down at the coming of the wind, cleaving the air along any line so the bird may then penetrate it. A second, longer note treats the mechanics of descent: a bird never falls backward because its centre of gravity lies toward the head, and it sinks along whichever line brings that centre nearest to the extremities of its span. He works through how the head, the folded wing, and the bent tail each shift the line of descent, and closes with the analogy that swimming upon water teaches men how birds move through the air. Two small sketches of a bird in two positions appear at the upper right.
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The tail-rudders that steer the bird against the wind
The rudders of a bird's wings are those that immediately set the bird above or below at the coming of the wind. With their small motion they cleave the air along whatever line, and through that opening the bird can then easily penetrate.
Descent governed by the centre of gravity
A bird never descends backward, because the centre of its gravity lies more toward the head than the tail. It descends along the line where that centre comes nearest the extremities of its breadth: drawing the head to the trunk with wings even and tail wide, it falls head first; folding one wing shifts the fall between that wing and the head; bending the tail or head toward one wing turns the descent oblique.
Swimming teaches men how birds fly
Leonardo closes the note with an analogy: swimming upon the water teaches men how birds do upon the air. The moving body finds its support and its balance in the same way in both fluids.